# Currying in JavaScript

Currying is a process in functional programming in which we can transform a function with multiple arguments into a sequence of nesting functions. It returns a new function that expects the next argument inline.

In other words, when a function, instead of taking all arguments at one time, takes the first one and return a new function that takes the second one and returns a new function which takes the third one, and so forth, until all arguments have been fulfilled.

That is, when we turn a function call `sum(1,2,3)` into `sum(1)(2)(3)`

The number of arguments a function takes is also called arity

`function sum(a, b) {    // do something}function _sum(a, b, c) {    // do something}`

Function `sum` takes two arguments (2-arity function) and `_sum` takes three arguments (3-arity function).

Curried functions are constructed by chaining closures by defining and immediately returning their inner functions simultaneously.

## Why it’s useful❓🤔

1. Currying helps we avoid passing the same variable again and again
2. It helps to create a higher order function

Currying transforms a function with multiple arguments into a sequence/series of functions each taking a single argument.

Example:
`function sum(a, b, c) {    return a + b + c;}`

This function takes three numbers, add the numbers and returns the result.

`sum(1,2,3); // 6`

As we see, function with the full arguments. Let’s create a curried version of the function and see how we would call the same function (and get the same result) in a series of calls:

```function sum(a) {    return (b) => {        return (c) => {            return a + b + c        }    }}
console.log(sum(1)(2)(3)) // 6```

We could separate this sum(1)(2)(3) to understand it better:

`const sum1 = sum(1);const sum2 = sum1(2);const result = sum2(3);console.log(result); // 6`

Let's get to know how it works:

We passed 1 to the `sum` function:

`let sum1 = sum(1);`

It returns the function:

`return (b) => {    return (c) => {        return a + b + c    }}`

Now, `sum1` holds the above function definition which takes an argument `b`.

We called the `sum1` function, passing in `2`:

`let sum2 = sum1(2);`

The `sum1` will return the third function:

`return (c) => {    return a + b + c}`

The returned function is now stored in `sum2` variable.

`sum2` will be:

`sum2 = (c) => {    return a + b + c}`

When `sum2` is called with 3 as the parameter,

`const result = sum2(3);`

It does the calculation with the previously passed in parameters: `a = 1`, `b = 2` and returns `6`.

`console.log(result); // 6`

The last function only accepts `c` variable but will perform the operation with other variables whose enclosing function scope has long since returned. It works nonetheless because of `Closure`

## Currying & Partial Application 👩🏻‍💻

Some might start to think that the number of nested functions a curried function has depends on the number of arguments it receives. Yes, that makes it a curry.

Let's take same `sum` example:

`function sum(a) {    return (b, c) => {        return a * b * c    }}`

It can be called like this:

```let x = sum(10);x(3,12);x(20,12);x(20,13);
// OR
sum(10)(3,12);sum(10)(20,12);sum(10)(20,13);```

Above function expects 3 arguments and has 2 nested functions, unlike our previous version that expects 3 arguments and has 3nesting functions.

This version isn’t a curry. We just did a partial application of the `sum` function.

Currying and Partial Application are related (because of closure), but they are of different concepts.

`Partial application transforms a function into another function with smaller arity.`

```function sum1(x, y, z) {    return sum2(x,y,z)}
// to
function sum1(x) {    return (y,z) => {        return sum2(x,y,z)    }}```
`function sum1(x) {    return (y) => {        return (z) => {            return sum2(x,y,z)        }    }}`

Currying creates nesting functions according to the number of the arguments of the function. Each function receives an argument. If there is no argument there is no currying.

To develop a function that takes a function and returns a curried function:

`function currying(fn, ...args) {    return (..._arg) => {        return fn(...args, ..._arg);    }}`

The above function accepts a function (fn) that we want to curry and a variable number of parameters(`…args`). The rest operator is used to gather the number of parameters after fn into `...args`.

Next, we return a function that also collects the rest of the parameters as `…_args`. This function invokes the original function fn passing in `...args` and `..._args` through the use of the spread operator as parameters, then, the value is returned to the user.

Now, we can use the above function to create curry function.

```function sum(a,b,c) {    return a + b + c}